Gas discharge devices are commonly provided by a gas discharge cavity in which a plurality of electrodes are attached thereto. An economic process for attaching the electrodes to the gas discharge cavity include, among others, the use of an indium material for sealing the electrode to the gas discharge cavity. The joint provided by indium has proven to be mechanically strong and usually, but not essentially, the joint provides a hermetic seal between any two articles. It is desirable that the indium seal be able to accommodate any differential in the thermal expansion between the two articles joined together.
In many laser devices, the gas discharge cavity is provided by a glass tube or block consisting of a glass, a ceramic, glass ceramic, or the like. On the other hand, the electrode generally consists of a metal such as aluminum, nickel-iron alloys, and the like. Therefore, the use of an indium seal in this application requires that the indium seal be somewhat malleable in its sealed condition in order to accommodate the different coefficients of expansion of two dissimilar materials. The use of indium in such an application is particularly described in U.S. Pat. No. 4,930,676, issued to McNaught et al, entitled "Joint Between Articles Of Materials Of Different Coefficients Of Thermal Expansion", and also in U.S. Pat. No. 4,273,282, issued to Norvell et al, entitled "Glass Or Ceramic-To-Metal Seals".
Laser applications of gas discharge devices require that the gas discharge cavity contain a lasing gas free from contaminants. Commonly, lasers utilize selected proportions of helium and neon gases. When components, such as electrodes and mirrors, are attached to the gas discharge cavity, the method of attachment must be such to minimize any contamination of the lasing gas during the life of the laser. Therefore, when using a joint material between the components and the gas discharge cavity, the joint material must minimally produce any outgassing into the gas discharge cavity, provide a hermetic seal, and be free from other contaminants which may be harmful to the mirrors which form, in part, a gas laser.
To satisfy the aforementioned requirements of attaching electrodes to gas discharge cavities for laser applications, an indium seal has been commonly selected, particularly as described in the aforementioned patents. Further, as is described in both of the above-referred to patents, the mounting surfaces of the articles to be attached, namely the gas discharge cavity and the electrode, are required to be "super clean." This is so since the indium seal adhesiveness to the cleaned parts is enhanced, and secondly so that the parts themselves do not contribute to any contamination of the lasing gas. A cleaning process for cleaning the articles to be sealed together is particularly described in both the McNaught et al patent and the Norvell et al patent.
One common practice in sealing an electrode to a laser block is to utilize preforms consisting of pre-cut lengths of indium wire. Great lengths are employed to clean these indium wire preforms prior to the sealing process. The McNaught et al patent considers the cleanliness of the indium wire preform to be so highly desired, that McNaught et al teaches and claims that an indium preform be produced "immediately" before providing the joint such that the indium preform wire is substantially free from impurities and minimally oxidized. In contrast, Norvell, et al, teaches cleaning the articles followed by applying an indium preform to one of the articles, followed by cleaning the combination of the article and preform before sealing the two articles together.
The Norvell et al patent is particularly applicable to joining an electrode made of aluminum or copper to a glass-ceramic material. In order to enhance the joint between these two dissimilar materials, the mounting surface of the electrode is first provided with a layer of gold followed by a layer of indium over the gold. The indium preform after being applied over the gold is heated to 175.degree. C. to form a gold-indium alloy between the electrode mounting surface and the exterior surface which is substantially indium. During the heating process, outgassing of any contaminants of the articles or indium is allowed to occur. In turn, the electrode with the indium layer is cooled, and subsequently cleaned before being pressed against the mounting surface of the glass-ceramic to form the joint.